– How do countries stop people from other countries entering theirs through the mountains, forests or water in the border?
They patrol the border. That’s pretty much the main way to ensure that that type of thing doesn’t happen.
What the other comments said you basically just guard them by patrolling and now more commonly with small portable drones as well for harder to travel areas.
Many land borders are actually not very actively guarded because climbing over mountains and ravines isn’t that easy which is why in those areas it’s just barren or wildlife only and barely anyone lives around the border. Areas you see having border guards and customs are easier areas for people to travel which is why there is more security and often fences and walls too to prevent illegal travelling.
Mostly by making it extremely difficult to stay in the country illegally.
They may patrol the border, but even the nk/sk dmz has holes. Everything else is basically theater.
But once you’re in, without a visa you can’t get a legit job or license, or housing, even healthcare. And if you ever get noticed by the police, they deport you. So its not a matter of getting in, it’s wanting/being able to stay.
It’s really hard to just wander through a large forest or mountain. Typically you’d want to follow a trail, but then patrolling a few trails is easy for border guards.
By actively guarding the border.
Obviously that isn’t perfect, and obviously people get through. But if a country really wants to strictly guard their border, all it’s takes is paying for more border guards and better border infrastructure.
That’s the only way to stop it, to have some sort of guard/law enforcement there to stop and arrest people crossing illegally